Push-to-talk (Ptt) communication links are used, in particular, in connection with mobile radio technology, in which case they are also referred to as push-to-talk-over-cellular (PoC) links since the corresponding Ptt devices are mobile radios. In this case, it is possible for a mobile radio subscriber to use its Ptt device to transmit a voice message to one or more recipients at the same time via a mobile radio interface, usually by operating a button. In this case, the voice data are usually distributed over the mobile radio communication network even while the sender is talking and are transmitted to the desired recipient(s). A PoC communication link is therefore similar to the known CB radio but has the big difference that the subscribers of a PoC link can be reached worldwide since the data are transmitted via a mobile radio network. The text below refers primarily to PoC links but the invention is not, in principle, restricted to use in the mobile radio sector.
PoC communication technically functions in a similar manner to Internet telephony (“Voice over IP”, VoIP). The subscribers first receive signaling by means of the SIP protocol (Session Initiation Protocol) and the voice data (“Talk Bursts”) are then transmitted using the RTP protocol (Real-Time Transport Protocol). In this case, an attempt is made to achieve the greatest possible standardization in order to ensure that the participating PoC devices are compatible. The “PoC Version 1.0” standard and a version 2.0, which are intended to define the fundamental properties of the communication protocols for establishing and operating the PoC communication link, currently exist, for instance.
According to currently customary industrial specifications, two different variants for establishing a PoC communication link (that is to say a PoC session) are described, said variants differing in the time at which a first PoC user can begin to talk after the PoC session has been established with a PoC server unit and the voice data are transmitted to the further PoC subscribers.
According to the so-called “late media mode”, the first PoC user, that is to say a first PoC client unit, is enabled to transmit data, for instance voice data, only when a PoC communication link has actually been established to a selected PoC user, which is also a PoC client unit, and this PoC user has also accepted the Ptt link request.
According to the second variant, which is also referred to as the “early media mode”, the PoC server unit has already enabled the first PoC user to transmit data even though there is not yet a communication link between the PoC server unit and the selected PoC subscriber. The first PoC subscriber therefore cannot be certain whether its data have been already received by the second PoC subscriber.
Additional difficulties arise when establishing a PoC session if data of different types are intended to be transmitted, that is to say not only voice data but also audio data (for example music files), video data, images, text messages or other textual data, as is also provided for in the PoC Version 2.0 standard. Difficulties may arise in this case, for instance, if not every one of the PoC clients can technically use each type of data or if a particular PoC client would not like to receive a particular type of data (for instance voice or audio data if the subscriber would like to operate its PoC device in the silent mode in particular situations). In these cases, only subgroups of types of data from a group of different types of data contained in the link request from a PoC client (the so-called “PoC initiator”) can be accepted by the other PoC clients. According to the prior art, the PoC initiator therefore does not have any opportunity whatsoever to determine whether the other PoC clients are ready to receive and what types of data they are ready to receive. It goes without saying that it would be possible, during a PoC session, to detect whether another subscriber can receive a particular type of data but the PoC session has already been created at this time and has already given rise to costs. It would be advantageous instead to be able to decide, even before a PoC session is established, whether a link to a subscriber is intended to be established.
This difficulty is mentioned in the PoC Version 2.0 standard without proposing a solution to it. Such a solution should be as simple as possible and should ensure that a PoC client is admitted to a PoC session only when it is ready to receive at least one type of data in common with all other PoC clients participating in the PoC session.